1. Field of the Invention
The present invention relates to a heat treatment apparatus for heating a thin plate-like precision electronic substrate such as a semiconductor wafer and a glass substrate for a liquid crystal display device (hereinafter referred to simply as a “substrate”) by irradiating the substrate with flashes of light.
2. Description of the Background Art
In the process of manufacturing a semiconductor device, impurity doping is an essential step for forming a pn junction in a semiconductor wafer. At present, it is common practice to perform impurity doping by an ion implantation process and a subsequent annealing process. The ion implantation process is a technique for causing ions of impurity elements such as boron (B), arsenic (As) and phosphorus (P) to collide against the semiconductor wafer with high acceleration voltage, thereby physically implanting the impurities into the semiconductor wafer. The implanted impurities are activated by the subsequent annealing process. When annealing time in this annealing process is approximately several seconds or longer, the implanted impurities are deeply diffused by heat. This results in a junction depth much greater than a required depth, which might present a problem in good device formation.
In recent years, attention has been given to flash lamp annealing (FLA) that is an annealing technique for heating a semiconductor wafer in an extremely short time. The flash lamp annealing is a heat treatment technique in which xenon flash lamps (the term “flash lamp” as used hereinafter refers to a “xenon flash lamp”) are used to irradiate the surface of a semiconductor wafer with flashes of light, thereby raising the temperature of only the surface of the semiconductor wafer doped with impurities in an extremely short time (several milliseconds or less).
The xenon flash lamps have a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of light emitted from the xenon flash lamps is shorter than that of light emitted from conventional halogen lamps, and approximately coincides with a fundamental absorption band of a silicon semiconductor wafer. Thus, when a semiconductor wafer is irradiated with flashes of light emitted from the xenon flash lamps, the temperature of the semiconductor wafer can be raised rapidly, with only a small amount of light transmitted through the semiconductor wafer. Also, it has turned out that the irradiation of a semiconductor wafer with flashes of light in an extremely short time of several milliseconds or less allows a selective temperature rise only near the surface of the semiconductor wafer. Therefore, the temperature rise in an extremely short time with the xenon flash lamps allows only the activation of impurities to be achieved without deep diffusion of the impurities.
Heat treatment apparatuses including such xenon flash lamps, in which pulsed light emitting lamps such as flash lamps are disposed on the front surface side of a semiconductor wafer and lamps that stay lit continuously such as halogen lamps are disposed on the back side thereof so that a desired heat treatment is performed using a combination of these lamps, are disclosed in U.S. Pat. No. 4,649,261 and International Publication No. WO 2003/085343. In these disclosed heat treatment apparatuses, the semiconductor wafer is preheated to a certain degree of temperature by the halogen lamps and the like, and is then raised in temperature to a desired treatment temperature by pulse heating from the flash lamps. Also, U.S. Patent Application Publication No. 2006/0291835 discloses an apparatus in which a semiconductor wafer placed on a hot plate is preheated to a predetermined temperature, and is then raised in temperature to a desired treatment temperature by irradiation with a flash of light from flash lamps.
When a semiconductor wafer is preheated by the hot plate as disclosed in U.S. Patent Application Publication No. 2006/0291835, a relatively uniform in-plane distribution of the wafer temperature is achieved by the precise temperature control of the hot plate. In particular, the hot plate disclosed in U.S. Patent Application Publication No. 2006/0291835 is divided into concentric zones which are individually temperature-controllable. This achieves the uniform in-plane distribution of the wafer temperature easily. On the other hand, the preheating using the halogen lamps as disclosed in U.S. Pat. No. 4,649,261 and International Publication No. WO 2003/085343 provides a process-related advantage in that the temperature of the semiconductor wafer is raised to a relatively high preheating temperature in a short time, but is prone to present a problem in that the temperature of a peripheral portion of the wafer is lower than that of a central portion thereof.
FIGS. 27A and 27B are graphs showing in-plane temperature distributions of a semiconductor wafer in a conventional heat treatment apparatus including flash lamps and preheating halogen lamps in combination. The graphs of FIGS. 27A and 27B show results obtained by preheating the semiconductor wafer implanted with impurities to a certain degree of temperature by the halogen lamps, irradiating the semiconductor wafer with flashes of light from the flash lamps to perform a pulse heating treatment, and then measuring a sheet resistance to determine a wafer temperature achieved. As shown in FIG. 27A, good temperature distribution uniformity is achieved in an inner region of the semiconductor wafer, rather than in a peripheral portion thereof, but the temperature falls rapidly in the peripheral portion.
When the amount of light directed from the halogen lamps onto the peripheral portion of the wafer for preheating is increased for the purpose of solving the problem of the temperature fall in the peripheral portion, the temperature of part of the peripheral portion rises, as shown in FIG. 27B, but the temperature distribution uniformity in the inner region, rather than in the peripheral portion, is impaired. Additionally, the temperature fall in an outermost edge portion of the semiconductor wafer still remains unsolved.